AUTO CONTROL FOR

3-PHASE MOTORS

D. DINESH

‘low’ (since J1 input of FF1 is already

‘high’ when the clock pulse arrives at CLK1 pin). Now, when the blue phase rises, the output of IC3 goes ‘high’, while

the output of IC2 is already ‘high’, result-ing in the output Q of FF2 goresult-ing ‘low’.

The above process repeats once dur-ing each 50Hz cycle. If Q outputs of both

FF1 and FF2 are ‘low’, the phase sequence is correct and both diodes D28 and D29 are in blocking mode. The base of tran-sistor T1 is pulled towards ground via re-Fig. 1: Schematic diagram of auto control for 3-phase motor

sistor R11 and transistor T1 starts con-ducting. As a result, IC5 is triggered and hence ‘sequence OK’ LED connected to pin 3 of IC5 via resistor R14, glows.

IC5 is a popular 555 timer wired as a retriggerable monoshot. Its time period is set at 25 milliseconds (approx.). If the monoshot is not retriggered within 25 mil-liseconds, the ‘sequence OK’ signal goes

‘low’. The circuit operates smoothly at

fre-quencies up to 42 Hz.

If any of the phase fails, the phase sequence is disturbed, resulting in the out-put of IC5 going ‘low’ and ‘sequence OK’

LED goes ‘off’. The LED status in relation to the phase sequence is shown in Table I.

The output of IC5 is also used for driving relay RL1 via transistor T2 (SL100).

Normally-open (N/O) contacts of re-lay RL1 are wired in series with ‘off’

switch of starter assembly as shown in the Fig. 1. Thus when phase se-quence is correct and the frequency is above 42 Hz, the relay is in energised state and it is feasible to switch on the starter by momentary energisation of relay RL2, whose N/O contacts are wired in parallel with the ‘on’ switch of starter assembly.

Auto-starter and current-sens-ing circuit. As soon as the phase se-quence is detected to be correct (as explained in the previous section), the output of IC5 goes ‘high’. This output, via resistor R15, is used to reset IC7 and enable IC6, besides acting as a clock for decade counter IC10.

IC6 is an NE555 timer wired in astable mode to provide clock pulses to decade counter CD4017 (IC7). Even-tually, when Q8 output of CD4017 (IC7) goes ‘high’, relay RL2 energises through transistor T9 (SL100). N/O contacts of RL2 are connected across

‘on’ switch of starter assembly, as stated earlier and the starter’s relay coil energises. The next clock pulse to IC7 deactivates relay RL2, but starter remains in ‘on’ state due to hold-on contact (the fourth contact of contactor in starter assembly). When Q9 (pin 11) of IC7 goes ‘high’, its CK pin 14 is muted due to conduction of transistor T8 (which pulls it to ground) to pre-vent further counting. The Q9 output of IC7 is also used in the motor ‘on’/

‘off’ timer circuit, explained later.

The supply to starter is connected through primaries of three small cur-rent transformers used for sensing the load in each phase. These trans-formers can be constructed using com-mon EI laminations generally used for power transformers. Core number 23 or 17 may be employed as per details given in Table II.

The secondaries of these transformers are connected to the current-sensing cir-cuit wired around transistors T3 through T5. If any phase goes ‘off’, it cuts off the corresponding transistor and thereby pro-vides forward bias to transistor T6.

The outputs of transistors T3 through Fig. 2: Actual-size, single-sided PCB layout for the circuit

T5 are wired-OR via diodes D15, D16, and D17. Any excessive increase in load cur-rent (overload) results in forward biasing of transistor T7. The excess current limit can be set with the help of preset VR1.

The conduction of transistors T6 and/

or T7 causes their common collector junc-tions to be pulled low. This ‘low’ signal is coupled to transistor T2 via diode D30.

As a result, relay RL1 deactivates to trip

the starter and thus stop the induction motor. The above conditions are summarised in Table III.

Motor on/off counter and latch.

Frequent start and stop operations sub-ject the motor to lot of fatigue due to heavy currents, which may damage the motor. In this circuit, automatic restart-ing of motor is limited to three attempts for each power ‘on’, by using another

de-cade counter CD4017 (IC10). It moni-tors each ‘on-off’ cycle of the motor by advancing the count of decade counter by one on every start.

The clock for IC10 is obtained from the output of IC5 via resistor R15. This point i.e. the junction of resistor R15 and diode D30 is also used as supply point for transistors T6, T7, T12 and T13 as also for reset pin of timer IC6.

On the third start, pin 7 (Q3) goes ‘high’

and transistor T13 gets forward biased.

As a result, CK pin 14 of IC10 is pulled low to stop any further clock to the decade counter, which thus gets latched and LED3 glows to indicate the latched state of the counter. Simultaneously, this ‘low’ signal causes transistor T2 to cut off and de-energise relay RL1. Thus the motor cannot restart automatically and only complete resumption of power can reset the latch.

Motor on-off timer. A timer is pro-vided to run the motor for a predeter-mined time. It counts run time of the motor and thereafter switches off the motor automatically. The signal from pin 11 (Q9) of IC7 is connected to the base of transistor T11 via resistor R38 (as referred in ‘auto-starter and curre-sensing circuit’). Thus the collector of transistor T11 goes ‘low’ to activate the oscillator circuit of CD4060 (IC8), while the motor is running. Prior to that, the oscillator circuit of CD4060 was inac-tive because its pin 11 was at logic ‘1’, being connected to +ve rails via resis-tors R39, R40 and diode D22. The fre-quency of oscillation is set by R-C net-work comprising 47µF capacitor C8 and resistor R42 in series with preset VR2.

A timing of either 30 minutes or 60 minutes can be selected with the help of switch S2 for the output of ‘on’/‘off’ timer to go from ‘low’ to ‘high’ state.The output from the pole of switch S2 is connected to the clock input of decade counter IC9. The outputs of IC9 go ‘high’ sequentially after 30/60-minute time intervals, depending on the selection made via switch S2. Thus multiples of 30-/60-minute basic timing can be selected with the help of 7-way rotary switch S3. (The 7-way rotary switch Fig. 3: Component layout for the PCB

may be substituted with decade thumb-wheel switch, if desired.)

The output available at the pole of rotary switch S3 goes ‘high’ after the se-lected duration to forward bias transistor T12, which, in turn, causes de-energisation of relay RL1. Also, when the selected run time is over, the oscilla-tor of IC8 (CD4060) gets inhibited because oscillator pin 11 of IC8 goes ‘high’ due to

the feedback from the pole of switch S3 via resistor R43 and diode D23. LED1 glows to indicate that run time is over.

To restart the motor, IC8 and IC9 can be manually reset by closing and then open-ing switch S1. The timer may be bypassed by keeping switch S1 closed.

The timer section requires very low

power in standby mode and is powered by four 1.5V cells as standby supply. A battery-low indicator is provided to warn the user about the low battery condi-tion.

Power supply.

The normal DC power supply for the circuit is provided by a small step-down transformer

X4 connected between R (red) phase and neutral, followed by rectifier and filter capacitor. The unregulated volt-age is used for operation of the relays, while the 5V regulated supply is used for the remaining circuit.